Process to prepare a lubricating base oil

ABSTRACT

Process to prepare a lubricating base oil having a saturates content of greater than 90 wt % and a viscosity index of greater than 90 starting from a vacuum distillate feedstock by performing the following steps: (a) contacting the vacuum distillate feedstock with an extraction solvent selective for extracting aromatic compounds from the vacuum distillate, (b) mixing the extracted oil obtained in step (a) with a feed comprising of more than 50 wt % wax or comprising of more than 80 wt % of paraffins and having a pour point greater than 0° C., (c) hydrotreating the blend obtained in step (b), (d) catalytically dewaxing the hydrotreated oil obtained in step (c) to obtained the lubricating base oil and (e) contacting the dewaxed oil with an aromatics saturation catalyst.

REFERENCE TO PRIOR APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/548,002 filed Feb. 26, 2004.

FIELD OF INVENTION

The invention is related to a process to prepare a lubricating base oilhaving a high saturates content and a high viscosity index starting froma vacuum distillate feedstock.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 5,976,354 discloses a process to make base oils having ahigh viscosity index (VI) and a low aromatics content from a waxydistillate feedstock. The majority of the aromatics are first removed bymeans of solvent extraction. The extracted oil is subsequently subjectedto a hydrotreating step followed by a catalytically dewaxing step andfinally to a aromatics saturation step.

A disadvantage of the process of U.S. Pat. No. 5,976,354 is that whenbase oils are to be made having a high VI the yield will dropdramatically. This is due to the more severe solvent extraction andhydrotreating process conditions which are required in order to arriveat the high VI base oil.

An alternative processing route to high viscosity index base oils is bystarting from a slack wax feed as for example described in EP-A-324528.In the disclosed process slack wax feed is firsthydrocracked/hydroisomerised and the resultant oil product is subjectedto a catalytic dewaxing step followed by a hydrotreating step.

A disadvantage of the process described in EP-A-324528 is that althoughhigh VI base oils are prepared the yield on feed is still not high.

EP-A-921184 discloses a process in which a blend of a waxy distillatefeedstock and a Fischer-Tropsch wax consisting predominately of normalparaffins is subjected to a hydrocracking process. The hydrocrackedblend is subsequently subjected to a solvent dewaxing step. It was shownthat higher VI base oils are obtained using this blended feedstock.However the yield of base oils calculated on the total feed was lowerwhen starting from the blend of mineral waxy distillate andFischer-Tropsch derived wax.

U.S. Pat. No. 6,576,120 describes a process wherein a partly isomerisedFischer-Tropsch wax is catalytically dewaxed using a Pt-ZSM-5/silicacontaining catalyst to a base oil having a pour point of −30° C. and aVI of 151.

U.S. Pat. No. 6,080,301 and U.S. Pat. No. 6,165,949 disclose base oil asprepared from a Fischer-Tropsch wax and their use in lubricantformulations.

SUMMARY OF THE INVENTION

The object of the present invention is to make a base oil having a highsaturates content and a high VI in a high yield starting from a vacuumdistillate feedstock.

This object has been achieved with the following process. Process toprepare a lubricating base oil having a saturates content of greaterthan 90 wt % and a viscosity index of greater than 100 starting from avacuum distillate feedstock by performing the following steps:

-   -   (a) contacting the vacuum distillate feedstock with an        extraction solvent selective for extracting aromatic compounds        from the vacuum distillate,    -   (b) mixing the extracted oil obtained in step (a) with a feed        comprising of more than 50 wt % wax or comprising of more than        80 wt % of paraffins and having a pour point greater than 0° C.,    -   (c) hydrotreating the blend obtained in step (b),    -   (d) catalytically dewaxing the hydrotreated oil obtained in        step (c) to obtained the lubricating base oil and    -   (e) contacting the dewaxed oil with an aromatics saturation        catalyst.

DETAILED DESCRIPTION OF THE INVENTION

Applicants found that by mixing a waxy and or paraffinic feed to theextracted oil the yield to high VI base oil can be significantlyimproved. This is explained by the fact that the paraffins in the waxyfeed, for example, a Fischer-Tropsch wax, will enhance the VI. Thus theseverity of both the solvent extraction and the hydrotreatment may bereduced resulting in an overall higher oil yield.

The vacuum distillate feed to step (a) is suitably a fraction boiling inthe base oil boiling range. Such a feed is also referred to as a mineralwaxy distillate. The base oil boiling range boils suitably above 350 andmore typically above 370° C. From distillate feeds it is possible toprepare base oil products having a kinematic viscosity at 100° C. ofabove 2 cSt and typically between 2 and 15 cSt. Such distillate feedsare preferably obtained by distillation of a suitably mineral crudepetroleum source at atmospheric pressure conditions. The residue thusobtained is subsequently further distilled at vacuum pressure conditionsin to one or more waxy distillate fractions and a so-called vacuumresidue. These waxy distillate fractions can be suitably used as feed tostep (a). The feedstock preferably has an initial boiling point in therange of from about 500° F. (260° C.) to about 650° F. (343° C.) and a95% boiling point generally in the range of 800° F. (427° C.) to 950° F.(510° C.).

In step (a) aromatics are extracted by contacting the feedstock with asuitable extraction solvent. These processes are well known in base oilmanufacturing and are for example described in Lubricant Base Oil andWax Processing, Avilino Sequeira, Jr, Marcel Dekker Inc., New York,1994, Chapter 5. Suitable solvents are well known, as are processconditions. Common suitable solvents include N-methyl-pyrrolidone,furfural, phenol and sulphur dioxide. In the process of the presentinvention a less severe extraction is allowed, preferably less than 50wt % of the aromatics content of the waxy raffinate feedstock needs tobe removed from said waxy distillate feedstock in step (a).

In step (b) the extracted oil as obtained in step (a), which is poor inaromatics relative to the original feedstock is blended with a feedcomprising of more than 50 wt % wax or comprising of more than 80 wt %of paraffins and having a pour point greater than 0° C. The feedstockemployed in the process of the invention preferably contains greaterthan about 70% wax. The paraffinic feed more preferably has a pour pointof greater than 10° C. and even more preferably more than 30° C. Theparaffinic feed more preferably comprises of more than 80 wt % paraffinsand even more preferably more than 90 wt % paraffins, wherein theparaffins may be normal and/or iso-paraffins.

Examples of the above described feed are synthetic oils such as preparedby the Fischer-Tropsch synthesis process, high pour pointpolyalphaolefins, foots oils, synthetic waxes such as normalpolyalphaolefin waxes, slack waxes, deoiled waxes and microcrystallinewaxes. Foots oil is prepared by separating oil from for example a slackwax. The isolated oil is referred to as foots oil.

A preferred waxy feed are slack waxes having a wax content of above 50wt % and more preferably above 70 wt %. A preferred paraffinic feed arethose obtained in the Fischer-Tropsch process. More preferably a partlyisomerised Fischer-Tropsch derived wax is used. By using such a partlyisomerised wax a less severe catalytic dewaxing is required which alsois beneficial for the final base oil yield. An additional advantage ofusing a partly isomerised Fischer-Tropsch wax is that it is more easilytransported from the typical remote Fischer-Tropsch location to the baseoil manufacturing location. The partly isomerised Fischer-Tropsch wax isalso referred to in this application as FT Waxy Raffinate. The FT WaxyRaffinate preferably has an initial boiling point of greater than 300°C. and a T95 wt % recovery boiling point of between 450 and 600° C.. Thepartly isomerised wax is different from a normal-paraffin wax in that ithas a relatively large oil content and a relatively smaller wax content,wherein the wax content is indicative for the presence of normalparaffins.

The wax content as used in this description is measured by solventdewaxing at −27° C. in MEK/Toluene. The wax content of the FT WaxyRaffinate is preferably between 10 and 80 wt %, more preferably between20 and 60 wt % and even more preferably below 40 wt %. The FT WaxyRaffinate is suitably prepared by hydrocracking a wax as obtained in theFischer-Tropsch synthesis reaction and recovering the above distillateor residual fraction from the effluent of the hydrocracking step.Examples of illustrative processes to prepare such a partly isomerisedFischer-Tropsch wax are disclosed in WO-A-02070630 and in EP-A-0668342.An example of a commercially available FT Waxy Raffinate is SMDS WaxyRaffinate as obtainable from Shell MDS (Malaysia) Sdn Bhd.

The content of the paraffinic and/or waxy feed component in the totalfeed to step (c) is preferably between 5 and 50 wt %, more preferablybetween 5 and 30 wt % and even more preferably between 5 and 15 wt %. Ithas been found that even with such relatively small amounts ofparaffinic and/or waxy feed a relatively large increase in base oilyield is achieved when intending to prepare the higher VI base oils. Itis also found that waxy distillate feedstocks, which are less suited tomake the high VI base oils, may nevertheless find application in thepresent invention.

In step (c) is generally operated to remove nitrogen and sulfur from theblend obtained in step (b). This process is referred to ashydrodesulfurization/ (302° C.) hydrodenitrogenation, HDS/HDN. Generallyspeaking, the HDS/HDN is conducted at a temperature in the range of fromabout 575° F. (302° C.). to about 780° F. (416° C.). Usually, thetemperature will be in the range of from about 600° F. to about 760° F.(404° C.). Preferably, the temperature will be in the range of about625° F. (329° C.). to about 730° F. (388° C.). Hydrogen will generallybe present at a hydrogen partial pressure in the range of from about 150psig to about 3500 psig, and total pressure will generally be in therange of from about 200 psig to about 4,000 psig. Usually, hydrogenpartial pressure will be in the range of from about 350 psig to about1400 psig and a total pressure will be in the range of from about 400psig to about 1500 psig.

The weight hourly space velocities (WHSV) is suitably in the range offrom 0.1 to 10 kg of oil per litre of catalyst per hour (kg/l/hr), andpreferably from 0.5 to 5 kg/l/hr, more preferably from 0.5 to 2.0kg/l/hr and hydrogen to oil ratios in the range of from 100 to 2000litres of hydrogen per litre of oil.

A preferred catalyst having HDS/HDN activity under these conditionsgenerally, a non-noble-metal-containing HDS/HDN catalyst. SuitableHDS/HDN catalysts generally comprise alumina or silica alumina and carryGroup VIII and/or Group VIB metals as the catalytically active agent.Most preferably, the catalytically active HDS/HDN agent is selected fromthe group consisting of nickel/molybdenum, cobalt/molybdenum andnickel/tungsten.

The Group VIII component generally comprises about 0.1 to about 30% byweight, preferably about 1 to about 15% by weight of the final catalyticcomposite calculated on an elemental basis. The Group VIB componentcomprises about 0.05 to about 30% by weight, preferably about 0.5 toabout 15% by weight of the final catalytic composite calculated on anelemental basis.

The hydrogenation components of the HDS/HDN catalyst will most likely bepresent in the oxide form after calcination in air and may be convertedto the sulfide form if desired by contact at elevated temperatures witha reducing atmosphere comprising hydrogen sulfide, a mercaptan or othersulfur containing compound. It is preferred that the catalyst(s) used inthe HDS/HDN zone is essentially free of any noble metal such as platinumor palladium. Further disclosure of possible catalysts and operatingconditions are described in detail in the afore-mentioned U.S. Pat. No.5,976,354, which publication is incorporated by reference.

Step (d) is performed in the presence of a suitable dewaxing catalysts,which catalysts are preferably heterogeneous catalysts comprising amolecular sieve and optionally in combination with a metal having ahydrogenation function, such as the Group VIII metals. Molecular sieves,and more suitably intermediate pore size zeolites, have shown a goodcatalytic ability to reduce the pour point of the base oil precursorfraction under catalytic dewaxing conditions. Preferably theintermediate pore size zeolites have a pore diameter of between 0.35 and0.8 nm. Suitable intermediate pore size zeolites are ZSM-5, ZSM-12,ZSM-22, ZSM-23, SSZ-32, ZSM-35 and ZSM-48, of which ZSM-12, ZSM-23,ZSM-22, SSZ-32 are most preferred because of their additionalisomerisation selectivities. Another preferred group of molecular sievesare the silica-aluminaphosphate (SAPO) materials of which SAPO-11 ismost preferred as for example described in U.S. Pat. No. 4,859,311.ZSM-5 may optionally be used in its HZSM-5 form in the absence of anyGroup VIII metal. The other molecular sieves are preferably used incombination with an added Group VIII metal. Suitable Group VIII metalsare nickel, cobalt, platinum and palladium. Examples of possiblecombinations are Ni/ZSM-5, Pt/ZSM-23, Pd/ZSM-23, Pt/ZSM-48 andPt/SAPO-11. Further details and examples of suitable molecular sievesand dewaxing conditions are for example described in WO-A-9718278, U.S.Pat. No. 5,053,373, U.S. Pat. No. 5,252,527 and U.S. Pat. No. 4,574,043.

The dewaxing catalyst suitably also comprises a binder. The binder canbe a synthetic or naturally occurring (inorganic) substance, for exampleclay, silica and/or metal oxides. Natural occurring clays are forexample of the montmorillonite and kaolin families. The binder ispreferably a porous binder material, for example a refractory oxide ofwhich examples are: alumina, silica-alumina, silica-magnesia,silica-zirconia, silica-thoria, silica-beryllia, silica-titania as wellas ternary compositions for example silica-alumina-thoria,silica-alumina-zirconia, silica-alumina-magnesia andsilica-magnesia-zirconia. More preferably a low acidity refractory oxidebinder material which is essentially free of alumina is used. Examplesof these binder materials are silica, zirconia, titanium dioxide,germanium dioxide, boria and mixtures of two or more of these of whichexamples are listed above. The most preferred binder is silica.

A preferred class of dewaxing catalysts comprise intermediate zeolitecrystallites as described above and a low acidity refractory oxidebinder material which is essentially free of alumina as described above,wherein the surface of the aluminosilicate zeolite crystallites has beenmodified by subjecting the aluminosilicate zeolite crystallites to asurface dealumination treatment. These catalysts may be advantageouslyused because they allow small amounts of sulphur and nitrogen in thefeed. A preferred dealumination treatment is by contacting an extrudateof the binder and the zeolite with an aqueous solution of afluorosilicate salt as described in for example U.S. Pat. No. 5,157,191or US-B-6,576,120. Examples of suitable dewaxing catalysts as describedabove are silica bound and dealuminated Pt/ZSM-5, silica bound anddealuminated Pt/ZSM-23, silica bound and dealuminated Pt/ZSM-12, silicabound and dealuminated Pt/ZSM-22, as for example described inUS-B-6,576,120 and EP-B-832171.

Preferred catalysts have a content of molecular sieve of between 5 and40 wt %. The average crystal size of the molecular sieve is preferablysmaller than 0.5 μm and more preferably smaller than 0.1 μm asdetermined by the well-known X-ray diffraction (XRD) line broadeningtechnique using the high intensity peak at about 20.9 2-theta in the XRDdiffraction pattern.

Catalytic dewaxing conditions are known in the art and typically involveoperating temperatures in the range of from 200 to 500° C., suitablyfrom 250 to 400° C., hydrogen pressures in the range of from 10 to 200bar, preferably between 20 and 100 bars. The weight hourly spacevelocities (WHSV) is suitably in the range of from 0.1 to 10 kg of oilper litre of catalyst per hour (kg/l/hr), and preferably from 0.2 to 5kg/l/hr, more preferably from 0.5 to 3 kg/l/hr and hydrogen to oilratios in the range of from 100 to 2,000 litres of hydrogen per litre ofoil.

The dewaxed oil is contacted in step (e) with an aromatics saturationcatalyst under aromatics saturation conditions. The aromatics saturationcatalyst comprises preferably. Generally the aromatics saturationcatalyst comprises oxides of platinum and palladium supported on analumina matrix. To provide selectivity for aromatic molecules, thematrix usually contains dispersed zeolite which has a pore size forpreferentially reacting aromatic molecules. Generally, only smallamounts of platinum and palladium are used. The aromatics saturationcatalyst will generally contain in the range of from about 0.1 wt % toabout 5 wt % platinum and in the range of from about 0.1 wt % to about 5wt % palladium, based on elemental weight of metal.

Operating temperature conditions in step (e) suitably does not exceed350° C. and preferably is in the range of from 150 and 350° C., morepreferably from 180 to 300° C. The operating pressure may range from 10to 250 bar and preferably is in the range of from 20 to 175 bar. TheWHSV may range from 0.1 to 10 kg of oil per litre of catalyst per hour(kg/l.h) and suitably is in the range from 0.5 to 6 kg/l.h.

Hydrotreating the blend of solvent refined waxy distillates and theparaffinic and/or waxy feed, for example FT Waxy Raffinate, to removethe sulfur and nitrogen in step (c) can be achieved at about the samehydroprocessing conditions (pressure, temperature, space velocity,hydrogen circulation rate) as selective dewaxing in step (d). A highdegree of aromatic saturation in step (e) can also be achieved at thesesame hydroprocessing conditions. This makes it feasible to hydrotreat,dewax and remove the majority of feed aromatics in three sequentialreactors in a reactor train forming a process unit as illustrated indetail in the afore mentioned U.S. Pat. No. 5,976,354. If desired, thelast two steps can be incorporated in the same process unit, e.g. thesame reactor vessel.

The desired lubricating base oil having the high viscosity index ofespecially between 100 and 135 is recovered from the effluent of step(e). Such recovery suitably involves fractionation of the effluent toobtain a gaseous fraction and at least one liquid fraction as thelubricating base oil product. Fractionation can be attained byconventional methods, such as by distillation of the effluent from thesecond reaction zone under atmospheric or reduced pressure. Of these,distillation under reduced pressure, including vacuum flashing andvacuum distillation, is most suitably applied. The cutpoint(s) of thedistillate fraction(s) is/are selected such that each product distillaterecovered has the desired viscosity, viscosity index and pour point forits envisaged application.

The base oils as produced preferably have a pour point of below −18° C.and more preferably below −27° C.. The kinematic viscosity at 100° C. ispreferably between 2 and 15 cSt. The viscosity index is preferably above90 and more preferably between 100 and 135. The saturates content ispreferably above 90 wt %, more preferably above 95 wt % and even morepreferably above 98 wt %. The sulphur content is below 100 ppmw,preferably below 50 ppmw, more preferably below 20 ppmw.

The invention will be illustrated by means of a calculated example. Inthis example it is aimed at preparing a base oil having a saturatescontent of 98 wt % and the viscosity index of 115-118 by means of aprocess according to the main claim and by means of the same process butwithout blending any FT Waxy Raffinate to the solvent extracted oil. TheFT Waxy Raffinate used in this simulation is the Shell MDS WaxyRaffinate having the properties in Table 1. The content of Shell MDSWaxy Raffinate in the blend is 10 wt %. TABLE 1 Shell MDS Waxy RaffinateFlash point (° C.) 225 Wax content 15 wt % Congealing point (° C.)  42When processing feedstock (i) alone a rather severe solvent extractionand hydrotreating step has to be performed in order to achieve a VIvalue of >115. As a base case the yield of these combined steps wasabout 60 wt % on feed. The dewaxing yield was between 80 and 85 wt %resulting in an overall yield of between 48 and 52 wt %.

When processing according to the invention we observed that a lesssevere solvent extraction/hydrotreatment is required in order to arriveat the same base oil quality. This results in an oil yield in theextraction/ hydrotreating steps of between 75 and 80 wt %. Because somewax is introduced via the waxy feed, and less wax if the FT waxyRaffinate is used, a slightly less selective dewaxing step is expectedhaving a oil yield of between 70 and 75 wt %. The overall oil yield wasbetween 63-70wt % which is considerably higher than when no FT WaxyRaffinate is added to the feed of the hydrotreating step.

1. A process to prepare a lubricating base oil having a saturatescontent of greater than 90 wt % and a viscosity index of greater than 90starting from a vacuum distillate feedstock by performing the followingsteps: (a) contacting the vacuum distillate feedstock with an extractionsolvent selective for extracting aromatic compounds from the vacuumdistillate feedstock, (b) mixing the extracted oil obtained in step (a)with a feed comprising of more than 50 wt % wax or comprising of morethan 80 wt % of paraffins and having a pour point greater than 0° C.,(c) hydrotreating the blend obtained in step (b), (d) catalyticallydewaxing the hydrotreated oil obtained in step (c) to obtained thelubricating base oil and (e) contacting the dewaxed oil obtained in step(d) with an aromatics saturation catalyst to yield the lubricating baseoil.
 2. A process according to claim 1, wherein the feed comprising ofmore than 50 wt % of wax in step (b) is a slack wax.
 3. A processaccording to claim 1, wherein the feed comprising more than 80 wt %paraffins and a pour point of greater than 0° C. in step (b) is a partlyisomerised Fischer-Tropsch wax having a wax content of between 10 and 80wt %.
 4. A process according to claim 3, wherein the partly isomerisedFischer-Tropsch wax has a wax content of between 20 and 60 wt %.
 5. Aprocess according to claim 4, wherein the blend obtained in step (b)comprises between 5 and 50 wt % of the paraffinic and/or waxy feed.
 6. Aprocess according to claim 5, wherein the blend obtained in step (b)comprises between 5 and 30 wt % of the paraffinic and/or waxy feed.
 7. Aprocess according to claim 6, wherein step (c) is performed in thepresence of a hydrotreating catalyst comprising one of the followingcombinations of metals selected from the group consisting ofnickel/molybdenum, cobalt/molybdenum and nickel/tungsten.
 8. A processaccording to claim 1, wherein step (d) is performed in the presence of adewaxing catalyst comprising a binder, a Group VIII metal, a molecularsieve selected from the group consisting of ZSM-5, ZSM-12, ZSM-22,ZSM-23, SSZ-32, ZSM-35, ZSM-48 and SAPO-11.
 9. A process according toclaim 8, wherein the molecular sieve is selected from the groupconsisting of ZSM-12, ZSM-23, ZSM-22 and SSZ-32.
 10. A process accordingto claim 9, wherein the Group VIII metal is platinum.
 11. A processaccording to claim 10, wherein the content of molecular sieve in thedewaxing catalyst is between 5 and 40 wt %.
 12. A process according toclaim 11, wherein the average crystal size of the molecular sieve issmaller than 0.5 μm and more preferably smaller than 0.1 μm asdetermined by the X-ray diffraction (XRD) line broadening techniqueusing the high intensity peak at about 20.9 2-theta in the XRDdiffraction pattern.
 13. A process according to claim 12, wherein thelubricating base oil as obtained have a pour point of below −18° C. andmore preferably below −27° C.
 14. A process according to claim 13,wherein the lubricating base oil as obtained has a kinematic viscosityat 100° C. in the range of from 2 and 15 cSt.
 15. A process according toclaim 14, wherein the lubricating base oil as obtained has a viscosityindex in the range of from 100 and
 135. 16. A process according to claim15, wherein the lubricating base oil as obtained has a saturates contentof above 95 wt %.
 17. A process according to claim 16, wherein thelubricating base oil as obtained has a sulphur content of below 50 ppmw.